Radar is an acronym for Radio Detection and Ranging. The term “radio” refers to a radio frequency (RF) wave called a carrier of frequency fc that is modulated to form a radar pulse train. The detection and ranging part of the acronym refers to timing a delay between transmission of an RF pulse and its subsequent return. If the time delay is Δt, a range of the radar may be determined by the formula:R=cΔt/2where c=3×108 m/s and is the speed of light. The factor of two in the formula accounts for the return trip.
Referring now to FIG. 1, a radar burst comprises a plurality of radar pulses. A pulse width (PW) of a radar pulse is the duration of the radar pulse. A pulse repetition interval (PRI) is an interval between the start of one radar pulse and the start of a subsequent radar pulse. A pulse repetition frequency (PRF) is the number of radar pulses transmitted per second and is equal to the inverse of PRI.
Military organizations use radar communication systems. Until recently, military radar communication systems enjoyed nearly interference-free communication. In recent years, however, wireless network communications have proliferated. As a result, wireless network signals may interfere with military radar communications. Interference between publicly used wireless networks and military radar systems is undesirable for security reasons.
Based on the disclosures by the military organizations, IEEE has defined the IEEE 802.11h specification, which is incorporated herein by reference in its entirety. IEEE 802.11h attempts to limit wireless networks and wireless network devices from interfering with radar systems. Support for IEEE 802.11h is required in all IEEE 802.11a compliant access points and client stations to avoid interference with military radar. One of the techniques IEEE 802.11h uses to reduce radio interference is Dynamic Frequency Selection (DFS).
Generally, when a device that employs DFS detects other devices on the same radio channel, the device switches to another channel if necessary. Typically, an access point (AP) in a wireless network transmits beacons and informs the client stations that the AP uses DFS. When the client stations detect radar on a channel, the client stations notify the AP. Based on this information, the AP uses DFS to select the best channel for network communications that will not interfere with radar.
Some network devices, however, may falsely detect radar on a channel. For example, a client station may incorrectly conclude that a noise signal such as a signal generated by a microwave appliance or other device is a radar signal. The network will unnecessarily block the channel despite the fact that the detected signal is not a radar signal. As false detections increase, additional channels may be blocked and fewer channels will remain available for network communications. This can significantly degrade network performance.